A combination of hard and soft templating for the fabrication of silica hollow microcoils with nanostructured walls
© Rodriguez-Abreu et al; licensee Springer. 2011
Received: 26 October 2010
Accepted: 13 April 2011
Published: 13 April 2011
Hollow silica microcoils have been prepared by using functionalized carbon microcoils as hard templates and surfactant or amphiphilic dye aggregates as soft templates. The obtained materials have been characterized by electron and optical microscopy, nitrogen sorption and small angle X-ray scattering. The obtained hollow microcoils resemble the original hard templates in shape and size. Moreover, they have mesoporous walls (pore size ≈ 3 nm) with some domains where pores are ordered in a hexagonal array, originated from surfactant micelles. The obtained silica microcoils also show preferential adsorption of cationic fluorescent dyes. A mechanism for the formation of silica microcoils is proposed.
The use of templates or scaffolds is one of the main strategies for the fabrication of advanced materials with new structures at the nano and micro scales that have attracted considerable research effort over the past decades. Templates can be classified as 'hard' and 'soft'. Hard templates are usually solid-state materials with particular structure and morphology, whereas soft templates are generally in a fluid-like state.
Hard templating is a conceptually straightforward and highly effective method to prepare hollow structures that mimic and/or complement the original shape of the templates [1, 2], which usually consists of the following steps: 1. Preparation of hard templates; 2. Functionalization/modification of template surface; 3. Coating the templates with the target shell material; and 4. Selective removal of the templates to obtain hollow structures. Silica particles and polymer latex colloids belong to the group of materials commonly employed as hard templates.
On the other hand, soft templates such as supramolecular self-assemblies are a powerful tool for the bottom-up synthesis of nanomaterials [3–6], particularly mesoporous inorganic solids. In this approach, there is a cooperative interaction between self-assemblies and inorganic species that lead to structuration.
Carbon microcoils (CMCs) , with coil diameters in the order of micrometers, are a class of carbon materials with singular properties, such as mechanical elasticity , high hydrogen sorption  and electromagnetic wave absorption . Carbon microcoils also help to improve material properties when incorporated in hybrid composites , or as template for other materials [12, 13].The formation of a coiled structure is attributed to the fact that the crystal faces of the catalyst used for the synthesis show different activity (i.e. catalytic anisotropy) in terms of carbon growth .
Although there is some literature on the use of carbon nanotubes (CNT) as scaffolds for the preparation of silica nanotubes with different morphologies [14–16], carbon materials with a peculiar structure such as CMC has not been used in a combined hard and soft templating strategy to produce hierarchically ordered materials. In this context, we report the results on the use of such a method to fabricate nanoporous hollow silica microcoils and discuss the characterization of the obtained materials. The combination of soft and hard templating provides versatility for the preparation of materials with properties deriving from structuration at different scales.
Carbon microcoils were synthesized according to a previous publication . Hexadecyltrimethylammonium bromide (CTAB), tetraethylorthosilicate (TEOS), rhodamine B and fluorescein were supplied by Sigma-Aldrich (USA). A Perylenebis(dicarboximide) dye (referred herein as PDI) was synthesized according to the literature .Ultrapure water (resistivity = 18.2 MΩ/cm) was used in the experiments. All chemicals were used without further purification.
Preparation of silica samples
Surface functionalization of CMCs was carried out by oxidation following a method already reported . Functionalized CMCs with -COOH groups are referred herein as CMC-COOH. In a typical preparation of silica hollow coils by sol-gel reaction, CTAB or PDI is dissolved in NH3 (aq., 25%). Then, CMC-COOHs are dispersed in the mixture by ultrasonication. Finally, TEOS is added and the mixture is stirred with a magnetic stirring bar for 3 h at 70°C. The resulting precipitate is washed, filtered, dried and calcined in air for 6 h at 600°C (heating rate = 1°C/min), above the decomposition temperature of CMC-COOHs, as determined by thermogravimetric analysis.
Scanning electron microscopic (SEM) images were collected with a Hitachi TM-1000 (Japan) and with a Zeiss UltraPlus FESEM instrument (Germany). Transmission electron microscopic (TEM) images were taken with a Hitachi H-7000(Japan). Specimens were deposited on copper grids from ethanol dispersions. Fluorescence microscopic images were collected with a Nikon Eclipse TE2000-U(Japan); for the observation, samples were immersed in an aqueous dye solution for 1 h and then rinsed thoroughly to remove the non-adsorbed dye. Small angle X-ray scattering (SAXS) measurements were performed in an instrument equipped with a Kratky camera and a linear position sensitive detector, OED 50 M, both from MBraun (Austria). Measurements were carried out at 0.5 kW with radiation coming from a Siemens generator, model Krystalloflex 760 (Germany). Nitrogen sorption isotherms were determined using a Micromeritics TriStar 3000 instrument (USA). Samples were degassed at 200°C, and weighed prior to sorption experiments. The pore size distribution was determined by the Barret-Joyner-Halenda (BJH) method .
Results and discussion
The fluorescence emission properties of the PDI adsorbed on CMC-COOH were preserved after silica coating. Dispersions of the PDI-silica coils in ethanol gave two emission bands at 540 and 575 nm (see Figure S4 in Additional file 1). However, a change in the UV-vis spectrum was observed (see Figure S5 in Additional file 1). Neat PDI solutions in ethanol show two absorption maxima at 530 and 590 nm, whereas the dispersions of PDI-silica coils exhibited only one maximum at 520 nm and a shoulder at about 580 nm.
Hollow silica microcoils with mesostructured walls were prepared by using carbon microcoils and amphiphilic molecules as hard and soft templates, respectively, and both serve as porogens upon calcination. Cationic aggregates adsorb on functionalized CMCs and behave both as an anchor and porogen of silica. The mesopores originated from surfactant aggregates were either ordered hexagonally or had a disordered, worm-hole morphology. Since the obtained hollow silica microcoils have a negatively charged surface (as a result of synthesis conditions), they show advantages for preferentially trapping cationic molecules. The method described here can be used to prepare hollow microcoils of other oxides via sol-gel reaction.
small angle X-ray scattering
scanning electron microscopic
Transmission electron microscopic
Authors are grateful to Consejo Superior de Investigaciones Científicas (CSIC, Spain) and National Science Council (NSC, Taiwan) for research funding within the frame of the bilateral cooperation program (2008TW006, 2009TW0031). C.R-A. is also grateful to to the Ministerio de Ciencia e Innovación, Spain (Project CTQ2008-01979/BQU) for financial support. Authors thank Lucia Casal (Universitat de Barcelona, Spain) for his help in the synthesis of PDI dye, and Prof. Po-Da Hong and Prof. Shawn D. Lin (National Taiwan University of Science and Technology) for experimental support.
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